fix: prevent asset conflicts between React and Grid.js versions

Add coexistence checks to all enqueue methods to prevent loading
both React and Grid.js assets simultaneously.

Changes:
- ReactAdmin.php: Only enqueue React assets when ?react=1
- Init.php: Skip Grid.js when React active on admin pages
- Form.php, Coupon.php, Access.php: Restore classic assets when ?react=0
- Customer.php, Product.php, License.php: Add coexistence checks

Now the toggle between Classic and React versions works correctly.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

node_modules/postcss-safe-parser/README.md

@@ -0,0 +1,34 @@
+# PostCSS Safe Parser
+
+<img align="right" width="135" height="95"
+     title="Philosopher’s stone, logo of PostCSS"
+     src="https://postcss.org/logo-leftp.svg">
+
+A fault-tolerant CSS parser for [PostCSS], which will find & fix syntax errors,
+capable of parsing any input. It is useful for:
+
+* Parse legacy code with many hacks. For example, it can parse all examples
+  from [Browserhacks].
+* Works with demo tools with live input like [Autoprefixer demo].
+
+[Autoprefixer demo]: http://simevidas.jsbin.com/gufoko/quiet
+[Browserhacks]:      http://browserhacks.com/
+[PostCSS]:           https://github.com/postcss/postcss
+
+<a href="https://evilmartians.com/?utm_source=postcss">
+  <img src="https://evilmartians.com/badges/sponsored-by-evil-martians.svg"
+    alt="Sponsored by Evil Martians" width="236" height="54">
+</a>
+
+
+## Usage
+
+```js
+const safe = require('postcss-safe-parser')
+
+const badCss = 'a {'
+
+postcss(plugins).process(badCss, { parser: safe }).then(result => {
+  result.css //= 'a {}'
+})
+```